1. Field of the Invention
The present invention relates to a viewfinder optical system and an optical apparatus that uses the viewfinder optical system. More specifically, the present invention relates to an eyepiece lens for a viewfinder used in a video camera, a still camera, or a broadcast camera.
2. Description of the Related Art
Conventionally, an electronic viewfinder used in a camera uses an eyepiece lens for allowing an observer to observe through the viewfinder a magnified image of an image displayed on a cathode ray tube (CRT) screen or a liquid crystal display (LCD) screen provided within the camera. Because an electronic viewfinder is required to have a function for allowing the observer to observe a magnified large image, it is necessary for an electronic viewfinder to use a large image display surface, such as a CRT screen or an LCD screen. Otherwise, it is required to increase the magnification of the eyepiece lens.
However, if a large image display surface is used, a large-size viewfinder may become necessary. Accordingly, in this case, it is useful to increase the magnification of the eyepiece lens rather than to increase the size of an image display surface. In order to increase the magnification of an eyepiece lens, it is necessary to steepen the angle of incidence of light incident to the pupil of an observer by intensifying the refractive power of the eyepiece lens. To that end, it has been conventional practice to use a large number of positive lenses.
However, axial chromatic aberration and lateral chromatic aberration cannot be corrected by using only lenses having a positive refractive power. Therefore, it is useful to correct chromatic aberration by using a lens having a negative refractive power.
Accordingly, it is useful, as a configuration of an eyepiece lens that uses a small number of constituent lenses and assigned with a high magnification, to use a minimum number of negative lenses for correcting chromatic aberration, i.e., one negative lens, and two lenses having a positive refractive power, the number of which is more than that of the negative lens by one.
Japanese Patent Application Laid-Open No. 2003-161915 discusses an eyepiece lens whose distance from an image display surface to an observation surface of the eyepiece lens is short and having three lenses including, in order from the image display surface side, a lens having a positive refractive power, a lens having a negative refractive power, and a lens having a positive refractive power. Japanese Patent Application Laid-Open No. 2007-264179 discusses an eyepiece lens having the same lens configuration as that of the eyepiece lens discussed in Japanese Patent Application Laid-Open No. 2003-161915. However, the eyepiece lens discussed in Japanese Patent Application Laid-Open No. 2007-264179 has a short principal point distance. Accordingly, the eyepiece lens discussed in Japanese Patent Application Laid-Open No. 2007-264179 has a short distance from the image display surface to an eye point.
With respect to an eyepiece lens, because the diameter of the pupil of the observer is fixed, if the magnification of the optical system is increased on a proportional basis, the image is not necessarily proportionally magnified contrary to the case of using a photographic lens. For example, coma occurring on an eyepiece lens may become more recognizable as coma after passing through the diameter of the pupil of the observer as the diameter of the eyepiece lens becomes smaller. Accordingly, the smaller the size of an eyepiece lens is, the more accurately it is required to correct coma.
The eyepiece lens discussed in Japanese Patent Application Laid-Open No. 2003-161915 is a small-size eyepiece lens having a focal length of less than 20 mm and includes three constituent lenses provided very close to one another. Accordingly, in order to achieve a sufficiently high magnification, it becomes difficult to correct coma in the above-described conventional eyepiece lens.
In addition, an eyepiece lens is required to include a function for adjusting a diopter corresponding to the eyesight of an observer. However, in the eyepiece lens discussed in Japanese Patent Application Laid-Open No. 2007-264179, the principal point position exists at a position close to the eyepiece lens side due to a wide distance between the three constituent lenses and due to a strong negative refractive power of the negative lens on the image display surface side. Accordingly, the distance from the image display surface to the lens provided closest to the image display surface is short.
As a result, in adjusting a diopter by the entire eyepiece lens, the range of adjustment of the diopter may become narrow in the direction of the negative-sign diopter.